In order to reduce as much as possible an area in which terminal devices cannot wirelessly communicate, a plurality of base station devices are often installed such that their radio coverage areas are overlapped each other. In this case, a signal transmitted from a certain base station device might arrive at a terminal device existing in a cell of another base station device near the certain base station device, and become an interference signal for the terminal device.
It is well known that such an interference can be suppressed by beam forming. That is, a base station device performs beam forming such that a beam is directed to a terminal device existing in its own cell (referred to as “desired terminal”, hereinafter) while a null beam is directed to a terminal device existing in a cell of another base station device (referred to as “interference terminal” hereinafter). Thereby, a signal (interference signal) from the base station device is less likely to arrive at the interference terminal, and thus interference is suppressed (refer to Non-Patent Document 1 for beam forming).